1. Field of the Invention
The present invention relates to a method and an apparatus for driving image pick-up tubes and, more particularly, to a method and an apparatus for driving image pick-up tubes in the state in which the noises in a signal amplifying system were suppressed. The method and apparatus for driving image pick-up tubes according to the present invention are suitable for use in a color TV camera, a home-use video camera or the like.
2. Description of the Related Art
Various kinds of image pick-up tubes are known, and a photoconductive type image pick-up tube is typical among them. This type of image pick-up tube is characterized by using a photoconductive material in a target portion of the image pick-up tube. Such an image pick-up tube has been disclosed in, e.g., Japanese Kokai 49-24619 (published on Mar. 5, 1974).
According to such a photoconductive type image pick-up tube, a signal current is obtained by driving the image pick-up tube by applying a predetermined voltage to a target portion made of a photoconductive material. Therefore, it is necessary to apply and use a voltage such that the potential of a target electrode arranged in the target portion is set to the positive value to the potential of a cathode electrode. At this time, in the case of, e.g., a color camera using a plurality of image pick-up tubes, the voltage applying method is classified as one of two kinds, dependent on whether of the target electrode or the cathode electrode of each image pick-up tube is grounded. Thus, there is the cathode electrode grounding drive system in which when the voltages which are supplied between the target electrodes and the cathode electrodes of respective image pick-up tubes differ, the potentials on the sides of the cathode electrodes of all of the image pick-up tubes are set to 0 V, and the voltage specified by each image pick-up tube is supplied to each target electrode. The alternative is the target electrode grounding drive system in which the potentials on the sides of the target electrodes of all of the image pick-up tubes are set to 0 V, and the voltage is supplied from each cathode electrode driver to each cathode electrode so as to obtain the specified potential difference between the target electrode and the cathode electrode.
A total of three image pick-up tubes, for the red (R) channel, the green (G) channel, and the blue (B) channel, are generally used in a color camera or the like. Since each image pick-up tube needs to be driven under the conditions suitable for the respective color channel, in a color camera using a plurality of image pick-up tubes it is necessary to supply different voltages between the target electrode and the cathode electrode of each image pick-up tube.
Therefore, in general, either one of the foregoing two systems is selected and used in the color camera.
In the cathode electrode grounding drive system, the S/N (signal current to noise) ratio of the image pick-up tube is given by the following equation. ##EQU1## where, the symbols in equation (1) represent the following: e: charge of electrons,
G: gain of a target of the image pick-up tube, PA1 I.sub.p : incident photon current, PA1 F: excess noise factory, PA1 B: band of a signal amplifier, PA1 k: Boltzmann's constant, PA1 T: absolute temperature, PA1 C.sub.0 : sum of the stray capacitance of the image pick-up tube and the input capacitance of the first-stage amplifier, PA1 R: load resistance of the image pick-up tube, PA1 R.sub.eq : equivalent noise resistance of the first-stage FET.
Therefore, when the image pick-up tube is driven by applying a relatively high voltage between its target electrode and its cathode electrode, it is necessary to use a coupling capacitor with a high withstanding voltage adapted to block the DC components and take out only the signal current components from the load resistance of a target. Therefore, the first-stage FET, signal take-out electrode of the target, and the like cannot be arranged closely and stray capacitance C.sub.0 increases. Thus, there is a problem of reduction in the S/N ratio of the amplifier as will be obvious from the equation (1).
On the other hand, in the case of the target electrode grounding drive system, since no DC component is included in the signal current which is derived from the target electrode, the foregoing coupling capacitor with a high withstanding voltage is unnecessary. Therefore, since the stray capacitance decreases by only the amount corresponding to the coupling capacitor, this system is more advantageous in terms of the S/N ratio than the cathode electrode grounding drive system.
However, in the target electrode grounding drive system, since the cathode potentials cannot be commonly set, drivers to supply power sources to all of the electrodes of a plurality of image pick-up tubes are individually needed. Thus, a constitution of the drivers increases and the weight, size, and electric power requirement of the color camera increase, so that this system is undesirable for reduction in size and weight of the camera.
The image pick-up tubes of the blocking type structure which are at present most frequently used have operating characteristics such that the output signal current exhibits a saturation tendency to the target voltage, and the target gain is up to 1 under the ideal conditions. Namely, in the case of the conventional image pick-up tubes of the blocking type structure, the signal current barely depends on the voltage between the target electrode and the cathode electrode. Therefore, a color television camera using a plurality of such image pick-up tubes of the blocking type structure is generally used by applying the same predetermined voltage to each cathode electrode and each target electrode. As explained above, in a color TV camera using a system in which a common potential difference is set between each target electrode and each cathode electrode of a plurality of image pick-up tubes to thereby operate them, it is common that the image signal processor system of the camera is adjusted so as to obtain the best image when an object is photographed under the standard image pick-up condition, i.e., under the illumination of the color temperature within a range from 3000.degree. to 3200.degree. K.
However, if the camera which has previously been adjusted so as to be adapted to only a specific image pick-up environment is used under another image pick-up circumstance, or in the case where such a camera must be used under different image pick-up circumstances, the following inconveniences occur.
Namely, for example, if the color camera which has previously been adjusted as mentioned above is used in the outdoors in the daytime or evening where the color temperatures of the incident lights differ, the balance of the signal currents of each channel changes, so that an image having a good color reproducibility is not obtained. To avoid this, hitherto, a color temperature converting filter is inserted in an optical path of the optical system to thereby change the color temperature of the incident lights to the color temperature near 3000.degree. K. in a manner such that the balance of the signal current of each channel does not greatly deviate from a desired value, and thereafter the gain of the amplifier is again adjusted and the color camera is used.
Further, in the case of photographing an image in the evening or a dark image at a place where an enough illumination is not obtained, not only the color temperature of the incident light changes but also the input signal level of the amplifier of each channel decreases. Therefore, it is also necessary to increase the gain of the amplifier so as to obtain a predetermined input signal level and to correct the color reproducibility, thereby obtaining a video image.
As explained above, in the system in which a common potential difference is set between each target electrode and each cathode electrode of a plurality of image pick-up tubes, the gain of the amplifier needs to be adjusted. Therefore, the gain of the amplifier of at least one of the channels must be increased, causing a problem such that the noise of the amplifier becomes conspicuous by only the amount corresponding to the increased gain. Further, since the operating dynamic range changes with the increase in the gain of the amplifier, there is a problem that it is necessary to perform complicated readjustments of the video signal processor system of the camera, such as white balance, gamma balance, shading correction, and the like.
In addition, even if the cathode electrode grounding drive system or target electrode grounding drive system is used to set the different potential difference between each target electrode and each cathode electrode of a plurality of image pick-up tubes, a camera suitable for a practical use cannot be obtained because these drive systems still have the problem of the reduction in S/N ratio of the amplifier as mentioned above, complication and increase in size of the constitution of the power source drivers.